Electrostatic spray coating method and apparatus therefor



y 1969 w, s. PETTIGREW 3,442,688

ELECTROSTATIC SPRAY COATING METHOD AND APPARATUS THEREFOR Filed Nov. 15, 1964 Sheet of 2 psgsURE SOURCE 5 5 (a? 1/2 55 INVENTOR.

W22? fif /gyrezd May 6, 1969 w, s. PETTIGRE W 3,442,688

ELECTROSTATIC SPRAY COATING METHOD AND APPARATUS THEREFOR Filed Nov. 15, 1964 Sheet 2 of 2 PAINT SUPPLY POWER SUPPLY PAINT SUPPLY x POWER SUPPLY 1 PAINT SUPPLY POW ERT SUPPLY 1 IN VENTOR. 2 2/%m%/ w l BY TTORNEY United States Patent 3,442,688 ELECTROSTATIC SPRAY COATING METHOD AND APPARATUS THEREFOR William S. Pettigrew, Pleasant Ridge, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Nov. 13, 1964, Ser. No. 411,040 Int. Cl. Bb 15/04 U.S. Cl. 11793.4 13 Claims ABSTRACT OF THE DISCLOSURE A centrifugal paint atomizing device is surrounded by a shielding device and voltage is applied to the assembly in such a manner that the electrical field intensity at the edge of the atomizing device is sufficient to effect charging of the paint but insufficient to effect electrostatic atomization of the paint.

This invention relates to a method of and apparatus,

for electrostatic spray coating, and more particularly to such a method and apparatus wherein atomization of liquid ciating material is effected solely by mechanicalpreferably centrifugal-means and .wherein deposition of the coating material on the workpiece is effected at least in part by electrosttaic forces.

It is known in the prior art to atomize liquid coating material mechanically in the presence of an electrostatic field having an intensity capable of electrostatica'lly atomizing the coating material. For example, a spinning disc has been used to atomize liquid coating material in an arrangement wherein the coating material is applied to the disc near its axis of rotation and the material is atomized from the edge of the disc, the disc forming one terminus of intense high-voltage electrostatic field and the article or articles to be coated forming the other terminus of the field. If the disc is rotated relatively slowly, the electrostatic field will effect electrostatic atomization of the coating material at the periphery of the disc. At high speeds of disc rotation, the coating material is atomized centrifugally independently of the electrostatic field at the edge of the disc. However, regardless of the speed of disc rotation, the atomized particles are very fine, indeed often too fine to give a satisfactory coating, primarily because the particles may dry before they reach the article to be coated so that proper flow of the coating does not occur and a dry scaly coating results. If the disc is rotated slowly so that electrostatic atomization occurs, the atomized particles are very fine because very fine particles are characteristic of electrostatic atomization. On the other hand, if the disc is rotated at a high speed to achieve mechanical atomization, the particles also are very fine because of the high centrifugal force involved. The arrangements discussed above lack sufficicnt flexibility to enable the operator of the apparatus to obtain the proper particle size desired for certain applications, particularly where relatively coarse particles are desirable or where a relatively large spread or range in particles size is desired. Relatively large particles and a large range in particle size can be obtained by utilizing centrifugal atomization from a disc which is rotating relatively slowly and where the atomization is completely mechanical, i.e., is entirely independent of the electrostatic field. If such an arrangement is used, and in addition the paint is charged at the atomizing site, effective electrostatic deposition can also be obtained.

3,442,688 Patented May 6, 1969 "ice A further disadvantage of prior art arrangements wherein the rotating disc forms one terminus of the electrostatic field is that corona discharge occurs at the relatively sharp edge of the disc. This discharge, in the presence of the combustible solvent vapors from the coating material, gives rise to fire hazard since the discharge produces an area of air ionization which can start an are.

One object of this invention is to provide a method of and apparatus for electrostatic coating wherein an intense high-voltage electrostatic field is utilized for depositing the atomized material, but wherein there is no corona discharge and wherein the coating material is mechanically atomized.

Another object of the invention is to provide a method of and apparatus for electrostatic spray painting wherein the paint is charged at the atomizing site to facilitate its electrostatic deposition and the atomizing device is electrically shielded to prevent electrostatic atomization therefrom.

A further feature of the invention is that the arrangement is such that there is no corona discharge either at the shielding means or at the atomizing device.

And still another feature of the invention is that it provides a method of and apparatus for electrostatic spray coating wherein the fineness of atomization and the range of atomized particle size may be mechanically controlled independently of the electrostatic field.

Other features and advantages of the invention will be apparent from the specification and drawings wherein:

FIGURE 1 is a perspective view of an apparatus embodying the invention;

FIGURE 2 is an enlarged view through the spray head shown in FIGURE 1;

FIGURE 3 is a diagrammatic view showing a modified form of spray head;

FIGURE 4 is a diagrammatic view showing a modification in the electrical connections of the apparatus of FIGURE 3; and

FIGURE 5 is a diagrammatic representation of a further-modified form of apparatus.

FIGURE 1 illustrates an overhead, so-called loop conveyor 10. In the center of the loop is an atomizing head designated generally as 12 which is supported by a conventional reciprocating device 14. The reciprocator 14 is guided by rollers 16 and is driven cyclically up and down by any conventional form of reciprocating motor, not shown. As is well understood in the art, the reciprocating apparatus continuously and cyclicly moves the atomizing head 12 between upper and lower positions throughout a range dependent upon the vertical span which it is desired to paint. A series of workpieces or articles to be coated 18 are carried by the conveyor 10 and .pass around the atomizing head 12 in spaced relation to .the atomizing head. The conveyor drive may be of any conventional type and is not illustrated since such conveyors are well understood in the art. The conveyor 10 is made of metal and is grounded and, since the articles 18 are suspended from the conveyor by metallic brackets 20 and metallic wire straps 22, the articles 18 also are grounded.

As shown in detail in FIGURE 2, the atomizing head 12 comprises a saucer-shaped metal disc 24 rotatably journaled on a shaft 26 of the reciprocator by means of upper and lower metallic bearings 28 and 28 respectively. Electrostatic shielding means comprising a pair of annular shielding electrodes or rings 30 and 32 are supported concentrically with and respectively above and below the disc 24 for reciprocal movement with the disc. The upper ring 30 is supported by means of a metallic spider having a plurality of arms 34 extending radially from a hub 36 which is mounted on the reciprocator shaft 26 above the disc 24. Similarly ,the lower ring 32 is supported below the disc 24 by a metallic spider having a plurality of arms 38 which extend from a hub 40 mounted on the reciprocator shaft 26 below the disc 24. The rings 30, 32, in the embodiment shown, are circular and are concentric with the disc 24. If variations in field and deposition patterns are desired, the rings may be formed in elliptical or other non-circular shape, and they may be mounted oif center with respect to the disc. A variablespeed air motor 42 is mounted on one of the spider arms 38 by means of a bracket 44 and the motor rotatably drives the disc 24 by means of a belt 46 extending between a pulley 48 on the air motor shaft and a pulley 50 on a disc hub member 52 which carries the bearings 28, 28'. The air motor 42 is connected to an air pressure source indicated diagrammatically at 54 by a hose 56 which extends through the hollow reciprocator shaft 26 and which has a lower portion 56 connected to the air motor 42 adjacent the bottom end of said shaft.

Paint or other liquid coating material is fed to the upper surface of the disc 24 from a paint supply indicated diagrammatically at 58 by means of a tube 60 of flexible insulating material such as nylon. A high voltage source or power supply is indicated diagrammatically at 62. This power supply has one terminal (preferably the positive terminal) grounded and the other terminal electrically connected by a conductor 64 to the metallic hub 36. A portion 26' of the reciprocator 14 immediately above the metallic hub 36 is formed of electrically insulating material as nylon, but the lower portion of the reciprocator shaft 26, the spiders 34 and 38 and their hubs 36 and 40, the rings 30 and 32, and the disc 24 are metallic. Accordingly, all the metallic parts of the atomizing head 12 are electrically interconnected and are at the potential of the high voltage source 62. By this means, an electrostatic field is created between the charged atomizing head 12 and the grounded articles 18.

Principally, the electrostatic field extends between the grounded articles 18 and the shielding rings 30, 32 so that the field intensity adjacent the shielding rings is high; however, because of the fact that the circular cross section of each of the shielding rings 30, 32 is of substantial diameter, i.e., at least of the order of one-quarter inch and preferably of the order of one-half inch, the field intensity at the rings is insufficient to create a corona discharge. While the principal field extends between the shielding rings and the articles, a marginal portion of the field exists in the space between the rings 30, 32 and the edge of the disc 24. Because of the shielding action of the rings 30, 32, the intensity of the field at and immediately adjacent the disc 24 is very low compared to the intensity of the field at and immediately adjacent the shielding rings. The intensity of the field at the periphery of the disc is so low that it is not capable of effecting electrostatic atomization of the coating material and is not capable of creating or sustaining a corona discharge. However, since there is a field at and immediately adjacent the edge of the disc, coating material which is mechanically atomized from the disc is electrically charged.

The dimensions of the shielding rings 30, 32, the diameter of the disc 24, the spacing between the shielding rings and the disc, and the voltages utilized are, in combination, somewhat critical. These dimensions, distances, and voltages must be selected for any given apparatus to provide a combination meeting these criteria: there is no corona discharge from either the rings or the disc of the atomizing head; the field intensity at the edge of the disc is capable of charging the coating material but is not capable of electrostatically atomizing the coating material; and the electrostatic field between the shielding rings and the grounded workpieces is strong enough to effect electro- 4 static deposition of the particles of coating material which are mechanically atomized from the disc 24. An example of these design criteria for a specific apparatus are as follows:

Diameter of disc 24 inches 10 Shielding rings 30, 32

(copper tubing diameter) do Outside diameter of rings 30, 32 do 22 Spacing between the rings 30, 32 do.. 6 Distance from edge of rings to workpiece do 18 Voltage on spray head kv .100

Under the above conditions, atomization from the disc is purely centrifugal. However, there is a field at and adjacent the edge of the disc of sufficient intensity to charge the atomized particles as they leave the disc so that the benefits of electrostatic deposition are obtained. by varying the speed of rotation of the disc, particle size can be controlled and there is a relatively large range in particle sizes which is particularly desirable for some coating applications.

Another important advantage obtained with the apparatus described above is that laminar air flow is obtained over the paint surface on the rotating disc. It has been proposed to use a saucer-like or plate-type shielding electrode above or below the disc but the use of such a plate-like electrode results in turbulent air flow across the surface of the disc which, in turn, results in improper flow of paint across the surface of the disc. Such platelike electrodes may also cause undesirable air turbulence in the space outside the edge of the disc and between the shielding electrodes so that proper projection of the atomized particles through the electrostatic shield and toward the article to be painted is not obtained. Utilizing rings as the electrostatic shield and disposing these rings well outside the edge of the disc minimizes any interference with the natural laminar air flow across the face of the disc so that proper paint flow and proper charging of the paint droplets is obtained.

Because the atomization is caused solely by mechanical means, the particle size of the atomized coating material is independent of the intensity of the electrostatic field and, hence, the potential on the atomizing head may be varied to suit the desired character of the depositing field so long as the above-mentioned criteria are met. Hence, the dimensions of the disc and shielding rings may differ from one installation to another according to the size of the conveyor loop, the nature of the articles being coated, the type of coating material being used, and other considerations which may determine the size of atomizing head required. Similarly, in a given apparatus the fineness of the atomized particles may be varied by changing several factors, such as the speed of disc rotation, viscosity and surface tension of the coating material and the rate of supplying coating material to the disc. In this manner, great flexibility in the operation of the painting apparatus is attained because the size of the particles being independent of the electrical field.

In operation, air pressure is supplied to the air motor 42 to rotate the disc 24 at a speed chosen to produce particles of a desired size to obtain a suitable coating. Electrical potential is applied to the head 12 and coating material is supplied to the central portion of the disc 24. As the disc rotates, the coating material is uniformly spread over the upper surface of the disc and advances in a thin film to the disc edge where it is atomized solely by centrifugal force. The relatively weak electrostatic field adjacent the disc causes the disc and the coating material to become electrically charged so that the atomized coating material carries an electrical charge as it leaves the periphery of the disc. The atomized material is projected by centrifugal force between and past the shielding rings 30, 32 generally in the plane of the disc edge but the atomized particles are mechanically dispersed in a fan like pattern as they leave the disc edge. The shielding rings 30, 32, however, being of the same polarity as the charged particles, tend to repel the particles toward the plane of the disc edge, thereby squeezing the pattern or reducing the degree of dispersions compared to that which would obtain in the absence of the shielding rings.

As the particles enter the strong electrostatic field established between the rings 30, 32 and the article 18, they become subject to the forces of that field and are deposited on the articles 18 in'a manner well known to those skilled in the art of electrostatic coating. However, the vertical width of the spray pattern is relatively small due to the squeezing action of the shielding rings on the charged paint particles. An advantage of this eifect is that a smaller amount of overspray occurs at the top and the bottom of the vertical travel of a reciprocating spray head, thereby reducing paint loss at these extreme positions as compared to the case where a wide spray pattern is used. Another advantage of the narrow spray pattern is the reduction of the loss of solvent which occurs in the case of a wide spray pattern where the particles travel through a larger volume of air. Thus, the likelihood that some particles will be deposited on the article in a dry state is minimized.

FIGURE 3 shows diagrammatically a modified form of the invention. As in the apparatus of FIGURES 1 and 2, liquid coating material is mechanically atomized from a I spinning disc 24 and is mechanically projected through and past an electrostatic shield provided by upper and lower shielding rings 30 and 32 and toward a grounded workpiece 18 which is suspended by metallic supporting means from a grounded conveyor 10. The disc 24 is driven by a motor 42 and paint is supplied to the disc from a pain supply 58. A high voltage source or power supply 62 provides means for charging the spray head.

Often, it may be desirable to provide a lower potential on the disc 24 than exists on the shielding rings 30, 32 to obtain better control of the degree of paint charging at the disc. In FIGURE 3 this is accomplished by connecting the rings'30 and 32 directly to the power supply 62 through a low-resistance conductor and by connecting the disc to the power supply through a voltage-dropping resistor 70. Since there is a small current fiow from the disc due to the projection of charged particles therefrom, there will be a voltage drop across the resistor 70 and the potential at the disc will be lower than the potential at the rings 30, 32. Instead of using a voltage-dropping resistor, separate variable voltage power supplies may be utilized, one being connected to the rings and the other being connected to the disc so that the voltages may be adjusted to apply a lower voltage on the disc than is applied on the rings. In one arrangement where the parts of the system were dimensioned and arranged as described earlier herein, it

was found that the application of voltages of 100 kv. on

the rings and 78 kv. on the disc gave excellent results. In the structure of FIGURE 3, it is, of course, necessary to insulate the disc 24 from the rings 30, 32. This is 'ular structure of FIGURE 2, such insulation could be obtained by making the cages of the bearings 28 and 28 of nylon or other electrically insulating material.

FIGURE 4' shows a further modification of the electrical connections wherein the disc is electrically isolated from the rings and is floating electrically, being disconnected from the power supply. In FIGURE 4, the general arrangement is identical with the arrangements earlier described. The disc 24 is mounted between upper and lower shielding rings 30 and 32 and mechanically atomizes liquid coating material for electrostatic deposition on a grounded workpiece 18 which is suspended from a grounded conveyor 10. The disc is rotatably driven by a motor 42, paint is supplied to the disc from a paint supply 58, and the spray head is charged by a high voltage source or power supply 62. In this instance, the disc may be either ofmetallic or of insulating material, as plastic.

. 6 As pointed out above, in the particular structure of FIGURE 2, making the cages for the bearings 28 and 28' of nylon or other suitable electrically insulating material would provide the desirable insulation.

Since the disc is electrically floating, it picks up a voltage by means commonly referred to as electrical induction. The magnitude of the induced voltage is dependent upon the magnitude of the voltage applied to the rings 30 and 32 and the special relationship of the disc to the rings. Thus, the voltage existing on the disc is always lower than the voltage on the rings and the voltage on the disc may be controlled by varying the spacial relationship between the disc and the rings. As in the arrangements earlier described where the disc was charged by direct connection to the power supply, there is a marginal field existing between the shielding rings 30, 32 and the disc, even though the disc is charged by induction. While this marginal field is of insufiicient intensity to create a corona discharge or to effect electrostatic atomization, it is sufiicient to charge the atomized particles which leave the disc.

FIGURE 5 shows a further modification of the invention in which the form of shielding electrode is changed from the rings of FIGURES 1-4 to a grid composed of a plurality of pegs 76 which are mounted in spaced, parallel relationship on a conducting ring 78 and which preferably extend from the ring 78 through and beyond the plane of the disc 24 and on opposite sides thereof. In this embodiment of the invention, the disc may be directly connected to the power supply and maintained at the same potential as the grid 76; it may be directly connected to the power supply and maintained at a lower potential than the grid 76, or it may be insulated from the grid and be electrically floating, obtaining its charge through electrical induction.

The individual pegs 76, which form the grid, are designed to prevent formation of a corona discharge and to this end the individual pegs are preferably round in cross section and have a substantial diameter as, for example, a diameter of one-quarter inch or more. The top end of each peg in rounded so that there are no points or sharp corners which would tend to create a corona discharge upon the application of a high voltage to the grid.

In the specification and claims, the term mechanical atomization is not contemplated to include air atomization, although it is contemplated to include various types of mechanical atomization in addition to centrifugal atomization; as for example, hydraulic atomization, vibratory or so-called ultrasonic atomization, and the like.

I claim:

1. The method of electrostatically spray coating an article comprising supplying liquid coating material to a disc-like centrifugal atomizing device, centrifugally atomizing the coating material and projecting the atomized material toward the article, providing around the atomizing device a pair of annular shielding rings concentric with the axis of rotation of the atomizing device and disposed one on each sde of the plane of atomizatin of the coating material, and establishing between the shielding rings and the article an electrostatic field having its highest intensity at the shielding rings but being of insufficient intensity at said rings to create a corona discharge, and said field having a marginal portion of lesser intensity extending adj acent said atomizing device, said portion being of sufficient intensity to electrically charge the coating material but being of insufiicient intensity to effect electrostatic atomization of the coating material from the atomizing device.

2. The method of electrostatically spray coating an article comprising supplying liquid coating material to a centrifugal atomizing device, centrifugally atomizing the coating material and projecting the atomized material toward the article, providing around the atomizing device an electrostatic shield and establishing between the atomizing device and the article an electrostatic field having adjacent the atomizing device of field intensity sufficient to electrically charge the coating material and insufiicient to effect electrostatic atomization of the material and having a field intensity at the electrostatic shield greater than that at the atomizing device and insuflicient to effect a corona discharge.

3. The method of electrostatically spray coating an article which is spaced from a centrifugal atomizing device, comprising providing an electrostatic shield in the space between the article and the atomizing device and having portions located on opposite sides of the plane of atomization from said atomizing device, supplying liquid coating material to said atomizing device, centrifugally atomizing the coating material and projecting the atomized material past said shield *and toward the artcle, and establishing an electrostatic field in the space between the article and the electrostatic shield, said field having its highest intensity at said electrostatic shield but being of insufiicient intensity to create a corona discharge, and said field having a marginal portion of lesser intensity extending adjacent said atomizing device, said marginal portion being of suflicient intensity to electrically charge the coating material but being of insuflicient intensity to effect electrostatic atomization of the material from the atomizing device.

4. The method of electrostatically spray coating an article which is spaced from a mechanical atomizing device, comprising providing an electrostatic shield in the space between the article and the atomizing device, supplying liquid coating material to said atomizing device, mechanically atomizing the coating material and projecting the atomized material past said shield and toward the article, and establishing an electrostatic field in the space between the article and the electrostatic shield, said field having its highest intensity at said electrostatic shield but being of insufficient intensity to create a corona discharge, and said field having a marginal portion of lesser intensity extending adjacent said atomizing device, said marginal portion being of sufiicient intensity to elec trically charge the coating material but being of insuificient intensity to effect electrostatic atomization of the material from the atomizing device.

5. The method of electrostatically spray coating an article which is spaced from a mechanical atomizing device, comprising providing an electrostatic shield in the space between the article and the atomizing device, supplying liquid coating material to said atomizing device, mechanically atomizing the coating material and projecting the atomized material past said shield and toward the article, and applying voltages of the same polarity but of different magnitudes to said atomizing device and electrostatic shield to establish an electrostatic field in the space between the article and the atomizing decice, said field having its highest intensity at said electrostatic shield but being of insufficient intensity at said electrostatic shield to create a corona discharge, and said field having, adjacent said atomizing device, a lesser intensity which is sufficient to electrically charge the coating material but which is insufiicient to effect electrostatic atomization of the material.

6. Apparatus for electrostatically spray coating an article, comprising a centrifugal atomizing device spaced from the article, means for rotating said atomizing device, means for feeding liquid coating material at a controlled rate to said atomizing device for centrifugal atomization therefrom and projection generally along a plane toward said article, a pair of annular shielding electrodes arranged concentric with the axis of rotation of the atomizing device and disposed one on each side of and equal distances from the plane of atomization of the coating material, said shielding electrodes each being greater in diameter than the largest dimension on the centrifugal atomizing device and being located closer to said article than is the atomizing device, and means for establishing between the shielding electrodes and the article an electrostatic field having its highest intensity at the shielding electrodes having insuflicient intensity at said shielding electrodes to create a corona discharge and having a marginal portion of lesser intensity extending adjacent said atomizing device, of sufficient intensity to electrically charge the coating material at the atomizing device and of insufiicient intensity to effect electrostatic atomization of the material from the atomizing device.

7. Apparatus for electrostatically spray coating an article comprising centrifugal atomizing means spaced from the article, means for feeding liquid coating material at a controlled rate to the atomizing means, a pair of electrode rings between the atomizing means and the article, one on each side of the plane of the centrifugal atomizing means; each electrode ring having a large radius of curvature designed to inhibit a corona discharge, and means for establishing an electrostatic field between the rings and the article having an intensity adjacent the atomizing means incapable of effecting electrostatic atomization of the coating material and sufficient to electrically charge the coating material.

8. Apparatus for electrostatically spray coating an article comprising mechanical atomizing means spaced from the article, means for feeding liquid coating material to the atomizing means, electrostatic shielding means between the article and the atomizing means, said shielding means having a large radius of curvature designed to inhibit a corona discharge, and means for establishing an electrostatic field between the shielding means and the article having an intensity adjacent the atomizing means incapable of effecting electrostatic atomization of the coating material and sufiicient to electrically charge the coating material.

9. Apparatus for spray coating an article with liquid coating material comprising an atomizing head, means for supplying coating material to the head at a controlled rate, means for establishing an electrostatic field between the head and the article, said head including means for centrifugally atomizing the coating material and shielding means for maintaining the electrostatic field adjacent the atomizing means at an intensity not capable of causing electrostatic atomization but of an intensity capable of electrically charging the coating material.

10. Apparatus for electrostatically spray coating an article, comprising a centrifugal atomizing device spaced from the article, means for rotating said atomizing device, means for feeding liquid coating material at a controlled rate to said atomizing device for centrifugal atomization therefrom and projection generally along a plane toward said article, electrostatic shielding means between the article and the atomizing device, said shielding means having a large radius of curvature designed to inhibit a corona discharge, means for applying a high voltage to said electrostatic shielding means and a voltage of the same polarity but of lesser magnitude to said atomizing device to establish between the electrostatic shielding means and the article an electrostatic field having its highest intensity at the electrostatic shielding means but being of insufiicient intensity at said shielding means to create a corona discharge, and said field having a marginal portion of lesser intensity extending adjacent said atomizing device, said portion being of sufiicient intensity to electrically charge the coating material but being of insufiicient intesnity to effect electrostatic atomization of the coating material from the atomizing device.

'11. Apparatus for electrostatically spray coating an article, comprising a centrifugal atomizing device spaced from the article, means for rotating said atomizing device, means for feeding liquid coating material at a controlled rate to said atomizing device for centrifugal atomization therefrom and projection generally along a plane toward said article, electrostatic shielding means between the article and the atomizing device, said shielding means having a large radius of curvature designed to inhibit a corona discharge, means electrically insulating said atomizing device from said shielding means,

and means for applying a high voltage to said electrostatic shielding means and for establishing between the electrostatic shielding means and the article an electrostatic field having its highest intensity at the electrostatic shielding means but being of insutficient intensity at said shielding means to create a corona discharge, and having a marginal portion of lesser intensity extending to said atomizing device whereby said atomizing device assumes a potential of the same polarity but of lesser magnitude than the voltage on said electrostatic shielding means, said portion having suflicient'intensity to electrically charge the coating material but insufficient intensity to effect electrostatic atomization of the coating material from the atomizing device.

12. Apparatus for electrostatically spray coating an article, comprising a centrifugal atomizing device spaced from the article, means for rotating said atomizing device, means for feeding liquid coating material at a controlled rate to said atomizing device \for centrifugal atomization therefrom and projection generally along a plane toward said article, electrostatic shielding means between the article and the atomizing device, said shielding means having a large radius of curvature designed to inhibit a corona discharge and comprising a pair of annular shielding electrodes arranged concentric with the axis of rotation of the atomizing device and disposed one on each side of the plane of atomization of the coating material, means for applying a high voltage to said electrostatic shielding means and a voltage of the same polarity but of lesser magnitude to said atomizing dew'ce to establish between the electrostatic shielding means and the article an electrostatic field having its highest intensity at the electrostatic shielding means but being of insufficient intensity at said shielding means to create a corona discharge, and said field having a marginal portion of lesser intensity extending adjacent said atomizing device, said portion being of suflicient intensity to electrically charge the coating material but being of insuflicient intensity to efiYect electrostatic atomization of the coating material from the atomizing device.

13. Apparatus for electrostatically spray coating an article, comprising a centrifugal atomizing device spaced from the article, means for rotating said atomizing device, means for feeding liquid coating material at a controlled rate to said atomizing device for centrifugal atomization therefrom and projection generally along a plane toward said article, an electrostatic shielding grid between the article and the atomizing device, comprising a plurality of pegs arranged in spaced parallel relationship and extending through said plane and on opposite sides thereof, said pegs having a large radius of curvature designed to inhibit a corona discharge, means for applying a high voltage to said electrostatic shielding grid to establish between the electrostatic shielding grid and the article an electrostatic field having its highest intensity at the electrostatic shielding grid but being of insufiicient intensity at said shielding grid to create a corona discharge, and said field having a marginal portion of lesser intensity extending adjacent said atomizing device, said portion being of suificient intensity to electrically charge the coating material but being of insufficient intensity to effect electrostatic atomization of the coating material from the atomizing device.

References Cited UNITED STATES PATENTS 2,899,136 8/1959 Reindl 118626 X 3,219,013 11/1965 Pettigrew et al 11862 7 FOREIGN PATENTS 616,933 3/1961 Canada.

OTHER REFERENCES Sedlacsik Methods of Electrostatic Coating from 117- 93.42, Organic Finishing, October 1957, pp. 7-8.

ALFRED L. LEAVITI, Primary Examiner.

J. H. NEWSO'ME, Assistant Examiner.

US. Cl. X.R. 

